Two primary types of heart valve replacements or prostheses are known. One is a mechanical-type heart valve that uses a ball and cage arrangement or a pivoting mechanical closure supported by a base structure to provide unidirectional blood flow, such as shown in U.S. Pat. No. 4,306,319 to Kaster. The other is a tissue-type or “bioprosthetic” valve having flexible leaflets supported by a base structure and projecting into the flow stream that function much like those of a natural human heart valve and imitate their natural action to coapt against each other and ensure one-way blood flow.
In tissue-type valves, a whole xenograft valve (e.g., porcine) or a plurality of xenograft leaflets (e.g., bovine pericardium) typically provide fluid occluding surfaces. Synthetic leaflets have been proposed, and thus the term “flexible leaflet valve” refers to both natural and artificial “tissue-type” valves. Two or more flexible leaflets are mounted within a peripheral support structure that usually includes posts or commissures extending in the outflow direction to mimic natural fibrous commissures in the native annulus. Components of the valve are usually assembled with one or more biocompatible fabric (e.g., Dacron) coverings, and a fabric-covered sewing ring is provided on the inflow end of the peripheral support structure.
In most bioprosthetic-type valves, metallic or polymeric structure provides base support for the flexible leaflets, which extend therefrom. One such support is an elastic “support frame,” sometimes called a “wireform” or “stent,” which has a plurality (typically three) of large radius cusps supporting the cusp region of the flexible leaflets (i.e., either a whole xenograft valve or three separate leaflets). The ends of each pair of adjacent cusps converge somewhat asymptotically to form upstanding commissures that terminate in tips, each extending in the opposite direction as the arcuate cusps and having a relatively smaller radius. The support frame typically describes a conical tube with the commissure tips at the small diameter end. This provides an undulating reference shape to which a fixed edge of each leaflet attaches (via components such as fabric and sutures) much like the natural fibrous skeleton in the aortic annulus. One example of the construction of a flexible leaflet valve is seen in U.S. Pat. No. 5,928,281 to Huynh, et al. (Edwards Lifesciences, Corp., Irvine, Calif.), in which the exploded view of FIG. 1 illustrates a fabric-covered wireform 54 and a fabric-covered support stent 56 on either side of a leaflet subassembly 52.
Many other flexible leaflet valve configurations are known, including U.S. Pat. No. 6,086,612 to Jansen (Adiam Medizintechnik GmbH & Co. KG, Germany) which discloses a mitral valve prosthesis having a support housing with a large base ring (12) that bears two stays (18, 19) which substantially extend in the ring axis direction and are connected by curved walls for securing two flexible cusps (leaflets). The free ends of the stays form an inner support for the cusps. The base-ring has in the top view a closed, non-round shape with a common longitudinal axis (15) but two transverse half-axes (16, 17) of different sizes. The valve is symmetric about the combined transverse half-axes. The stays lie on the longitudinal axis and form the transition between the two halves of the valve. The less curved wall (13) carries a mural cusp having a smaller surface and a higher angle of inclination relative to the base ring base surface than the leaflet connected to the more curved wall (14). The material for the cusps (leaflets) can be synthetic resin foils known from the state of the art, preferably thermoplastic elastomers or synthetic resins with elastomeric properties such as a flexible polyurethane foil.
Another flexible leaflet valve configuration is disclosed in U.S. Pat. No. 6,171,335 to Wheatley, et al. (Aortech Europe Limited, Great Britain). This valve includes a generally annular frame with three posts and three scallops. The frame is tri-symmetric with an axis of symmetry defined by the axis of blood flow through the valve. Each leaflet has a truncated spherical surface adjacent to its free edge that is joined tangentially to a truncated conical surface. The leaflet surface is axi-symmetrical with the axis of symmetry being perpendicular to the axis of the valve frame and blood flow. The leaflets can comprise any biostable, biocompatible thermoplastic elastomer including but not limited to any polyurethane or silicone elastomer or any copolymer or blend based on these elements.
U.S. Pat. No. 6,613,086 to Moe, et al. (CarboMedics Inc., Austin, Tex.) discloses a tri-leaflet prosthetic cardiac valve with leaflets having an analytic shape in a selected position. The leaflets are connected to a valve body at attachment curves. The shape of the leaflet is selected from a set of geometries that can be represented mathematically. The attachment curve is selected to improve the durability of the tri-leaflet valve by moving the point of maximum loaded stress along the attachment curve away from the commissures. An inner wall of the valve body is given a non-circular shape near the attachment curve, the shape of the inner wall corresponding to the attachment curve.
Unfortunately, some proposed valves deteriorate quickly, and some inordinately restrict flow which undesirably reduces the amount of oxygen supplied to the body. The manufacturing process of tissue heart valves is very mature and complex from the quality control point of view, and only minimal improvements in valve durability have been achieved in recent years. Accordingly, despite much development work on heart valves in the past three decades, there remains a need for a more durable valve that permits more blood to flow.